Ascidiacea

Ascidiacea Temporal range: Cambrian "Stage 3" – today (but see text)[1] PreꞒ Ꞓ O S D C P T J K Pg N
Ciona intestinalis, commonly known as the vase tunicate or as a sea squirt
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Tunicata
Class:	Ascidiacea Blainville, 1824
Groups included
Aplousobranchia Lahille, 1887 Phlebobranchia Lahille, 1887 Stolidobranchia Lahille, 1886 † Cheungkongella ? Shu et al., 2001 †Ausiidae? † Ausia ? †Burykhia?
Cladistically included but traditionally excluded taxa
Thaliacea Nielsen, 1995 Appendicularia? Lahille, 1890

Ascidiacea, commonly known as the ascidians or sea squirts, is a

.

Ascidians are found all over the world, usually in shallow water with

]

There are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a zooid) forming large colonies.^[3]

Sea squirts feed by taking in water through a tube, the oral siphon. The water enters the mouth and pharynx, flows through mucus-covered gill slits (also called pharyngeal stigmata) into a water chamber called the atrium, then exits through the atrial siphon. ^{[citation needed]}

Some authors now include the thaliaceans in Ascidiacea, making it monophyletic.^[4]

Anatomy

Sea squirts are rounded or cylindrical animals ranging from about 0.5 to 10 cm (0.2 to 4 in) in size. One end of the body is always firmly fixed to rock, coral, or some similar solid surface. The lower surface is pitted or ridged, and in some species has root-like extensions that help the animal grip the surface. The body wall is covered by a smooth thick tunic, which is often quite rigid. The tunic consists of cellulose, along with proteins and calcium salts. Unlike the shells of molluscs, the tunic is composed of living tissue and often has its own blood supply. In some colonial species, the tunics of adjacent individuals are fused into a single structure.^[5]

The upper surface of the animal, opposite to the part gripping the substratum, has two openings, or siphons. When removed from the water, the animal often violently expels water from these siphons, hence the common name of "sea squirt". The body itself can be divided into up to three regions, although these are not clearly distinct in most species. The pharyngeal region contains the pharynx, while the abdomen contains most of the other bodily organs, and the postabdomen contains the heart and gonads. In many sea squirts, the postabdomen, or even the entire abdomen, are absent, with their respective organs being located more anteriorly.^[5]

As its name implies, the pharyngeal region is occupied mainly by the pharynx. The large buccal siphon opens into the pharynx, acting like a mouth. The pharynx itself is

The pharynx is surrounded by an atrium, through which water is expelled through a second, usually smaller, siphon. Cords of connective tissue cross the atrium to maintain the general shape of the body. The outer body wall consists of connective tissue, muscle fibres, and a simple epithelium directly underlying the tunic.^[5]

bryozoans (Triphyllozoon inornatum

).

Digestive system

The pharynx forms the first part of the digestive system. The endostyle produces a supply of

The esophagus runs downwards to a

Circulatory system

The heart is a curved muscular tube lying in the postabdomen, or close to the stomach. Each end opens into a single vessel, one running to the endostyle, and the other to the dorsal surface of the pharynx. The vessels are connected by a series of sinuses, through which the blood flows. Additional sinuses run from that on the dorsal surface, supplying blood to the visceral organs, and smaller vessels commonly run from both sides into the tunic.

Unusually, the heart of sea squirts alternates the direction in which it pumps blood every three to four minutes. There are two excitatory areas, one at each end of the heart, with first one being dominant, to push the blood through the ventral vessel, and then the other, pushing it dorsally.[5]

There are four different types of blood cell:

Nervous system

The ascidian central nervous system is formed from a plate that rolls up to form a neural tube. The number of cells within the central nervous system is very small. The neural tube is composed of the sensory vesicle, the neck, the visceral or tail ganglion, and the caudal nerve cord. The anteroposterior regionalization of the neural tube in ascidians is comparable to that in vertebrates.^[7]

Although there is no true brain, the largest ganglion is located in the connective tissue between the two siphons, and sends nerves throughout the body. Beneath this ganglion lies an exocrine gland that empties into the pharynx. The gland is formed from the nerve tube, and is therefore homologous to the spinal cord of vertebrates.^[5]

Sea squirts lack special sense organs, although the body wall incorporates numerous individual receptors for touch,

Life history

Almost all ascidians are

Solitary ascidians release many eggs from their atrial siphons;

]

As a general rule, the larva possesses a long tail, containing muscles, a hollow dorsal nerve tube and a notochord, both features clearly indicative of the animal's chordate affinities. But one group, the molgulid ascidians, have evolved tailless species on at least four separate occasions, and even direct development.^[9] A notochord is formed early in development and always consists of a row of exactly 40 cells.^[10] The nerve tube enlarges in the main body, and will eventually become the cerebral ganglion of the adult. The tunic develops early in embryonic life and extends to form a fin along the tail in the larva. The larva also has a statocyst and a pigmented cup above the mouth, which opens into a pharynx lined with small clefts opening into a surrounding atrium. The mouth and anus are originally at opposite ends of the animal, with the mouth only moving to its final (posterior) position during metamorphosis.^[5]

The larva selects and settles on appropriate surfaces using receptors sensitive to light, orientation to gravity, and

]

Direct development in ascidians

Some ascidians, especially in Molgulidae family, have direct development in which the embryo develops directly into the juvenile without developing a tailed larva.^[11]

Colonial species

Colonial ascidians reproduce both asexually and sexually. Colonies can survive for decades. An ascidian colony consists of individual elements called zooids. Zooids within a colony are usually genetically identical and some have a shared circulation.^[8]

Sexual reproduction

Different colonial ascidian species produce sexually derived offspring by one of two dispersal strategies – colonial species are either broadcast spawners (long-range dispersal) or

develop into microscopic larvae that may be carried great distances by oceanic currents. The larvae of sessile forms which survive eventually settle and complete maturation on the substratum- then they may bud asexually to form a colony of zooids.

The picture is more complicated for the philopatrically dispersed ascidians: sperm from a nearby colony (or from a zooid of the same colony) enter the atrial siphon and fertilization takes place within the atrium. Embryos are then brooded within the atrium where

embryonic development

takes place: this results in macroscopic tadpole-like larvae. When mature, these larvae exit the atrial siphon of the adult and then settle close to the parent colony (often within meters). The combined effect of short sperm range and philopatric larval dispersal results in local population structures of closely related individuals/inbred colonies. Generations of colonies which are restricted in dispersal are thought to accumulate adaptions to local conditions, thereby providing advantages over newcomers.

Trauma or predation often results in fragmentation of a colony into subcolonies. Subsequent zooid replication can lead to coalescence and circulatory fusion of the subcolonies. Closely related colonies which are proximate to each other may also fuse if they coalesce and if they are

histocompatible

. Ascidians were among the first animals to be able to immunologically distinguish self from non-self as a mechanism to prevent unrelated colonies from fusing to them and parasitizing them.

Fertilization

Sea squirt eggs are surrounded by a fibrous

physiological

and structural changes of development.

The dramatic rearrangement of egg cytoplasm following fertilization, called ooplasmic segregation, determines the dorsoventral and anteroposterior axes of the embryo. There are at least three types of sea squirt egg

Promotion of out-crossing

Ciona intestinalis is a hermaphrodite that releases sperm and eggs into the surrounding seawater almost simultaneously. It is self-sterile, and thus has been used for studies on the mechanism of self-incompatibility.[14] Self/non-self-recognition molecules play a key role in the process of interaction between sperm and the vitelline coat of the egg. It appears that self/non-self recognition in ascidians such as C. intestinalis is mechanistically similar to self-incompatibility systems in flowering plants.^[14] Self-incompatibility promotes out-crossing, and thus provides the adaptive advantage at each generation of masking deleterious recessive mutations (i.e. genetic complementation).^[15]

Ciona savignyi is highly self-fertile.^[16] However, non-self sperm out-compete self-sperm in fertilization competition assays. Gamete recognition is not absolute allowing some self-fertilization. It was speculated that self-incompatibility evolved to avoid inbreeding depression, but that selfing ability was retained to allow reproduction at low population density.^[16]

Botryllus schlosseri is a colonial tunicate able to reproduce both sexually and asexually. B. schlosseri is a sequential (protogynous) hermaphrodite, and in a colony, eggs are ovulated about two days before the peak of sperm emission.^[17] Thus self-fertilization is avoided, and cross-fertilization is favored. Although avoided, self-fertilization is still possible in B. schlosseri. Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23% vs. 1.6%).^[17] Also, a significantly lower percentage of larvae derived from self-fertilized eggs metamorphose, and the growth of the colonies derived from their metamorphosis is significantly lower. These findings suggest that self-fertilization gives rise to inbreeding depression associated with developmental deficits that are likely caused by expression of deleterious recessive mutations.^[15]

Asexual reproduction

Many colonial sea squirts are also capable of asexual reproduction, although the means of doing so are highly variable between different families. In the simplest forms, the members of the colony are linked only by rootlike projections from their undersides known as stolons. Buds containing food storage cells can develop within the stolons and, when sufficiently separated from the 'parent', may grow into a new adult individual.^[5][8]

In other species, the postabdomen can elongate and break up into a string of separate buds, which can eventually form a new colony. In some, the pharyngeal part of the animal degenerates, and the abdomen breaks up into patches of germinal tissue, each combining parts of the epidermis, peritoneum, and digestive tract, and capable of growing into new individuals.^[5]

In yet others, budding begins shortly after the larva has settled onto the substrate. In the family Didemnidae, for instance, the individual essentially splits into two, with the pharynx growing a new digestive tract and the original digestive tract growing a new pharynx.^[5]

DNA repair

Apurinic/apyrimidinic (AP) sites are a common form of DNA damage that inhibit DNA replication and transcription. AP endonuclease 1 (APEX1), an enzyme produced by C. intestinalis, is employed in the repair of AP sites during early embryonic development.^[18] Lack of such repair leads to abnormal development. C. intestinalis also has a set of genes that encode proteins homologous to those employed in the repair of DNA interstrand crosslinks in humans.^[19]

Ecology

The exceptional filtering capability of adult sea squirts causes them to accumulate

Over the last few hundred years, most of the world's

Sea squirts are the natural prey of many animals, including

pharmaceuticals

.

Evolution

Fossil record

Ascidians are soft-bodied animals, and for this reason, their fossil record is almost entirely lacking. The earliest reliable ascidians is Shankouclava shankouense from the Lower

They are also recorded from Lower Jurassic (Bonet and Benveniste-Velasquez, 1971; Buge and Monniot, 1972[24]) and the Tertiary from France (Deflandre-Riguard, 1949, 1956; Durand, 1952; Deflandre and Deflandre-Rigaud, 1956; Bouche, 1962; Lezaud, 1966; Monniot and Buge, 1971; Varol and Houghton, 1996^[25]). Older (Triassic) records are ambiguous.^[26] The representatives of the genus Cystodytes (family Polycitoridae) have been described from the Pliocene of France by Monniot (1970, 1971) and Deflandre-Rigaud (1956), and from Eocene of France by Monniot and Buge (1971), and lately from the Late Eocene of S Australia by Łukowiak (2012).^[27]

Phylogeny

The ascidians were on morphological evidence treated as sister to the

Tunicata	Aplousobranchia (ascidians) Appendicularia Stolidobranchia (ascidians) Phlebobranchia (ascidians) Thaliacea

Recent studies have suggested an alternate phylogeny, placing

Tunicata	Appendicularia Acopa Stolidobranchia (ascidians) Thaliacea Enterogona Phlebobranchia (ascidians) Aplousobranchia (ascidians)

Uses

Culinary

Various ascidians are used as food.

Microcosmus species from the Mediterranean Sea are eaten in France (figue de mer, violet), Italy (limone di mare, uova di mare) and Greece (fouska, φούσκα), for example, raw with lemon, or in salads with olive oil, lemon and parsley.

The piure (

.

Aboriginal people living around Botany Bay

, but is now used mainly for fishing bait.

Model organisms for research

Several factors make sea squirts good models for studying the fundamental developmental processes of

embryonic development

of sea squirts is simple, rapid, and easily manipulated. Because each embryo contains relatively few cells, complex processes can be studied at the cellular level, while remaining in the context of the whole embryo. The eggs of some species contain little yolk and are therefore transparent making them

embryogenesis from beginning to end.^[35]

Sea squirts are also valuable because of their unique

cDNAs have been sequenced and are available to support further analysis of gene expression, which is expected to provide information about complex developmental processes and regulation of genes in vertebrates. Gene expression in embryos of sea squirts can be conveniently inhibited using Morpholino oligos.^[37]

References

Citations

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2. ^ Gittenberger, A.; Shenkar, N.; Sanamyan, K. (2015). "Ascidiacea". In: Shenkar, N.; Gittenberger, A.; Lambert, G.; Rius, M.; Moreira Da Rocha, R.; Swalla, B. J.; Turon, X. (2017). Ascidiacea World Database. Accessed through: World Register of Marine Species on 2017-09-15.
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4. ^ ^{a b} Brusca, Richard C.; Giribet, Gonzalo; Moore, Wendy (2023). Invertebrates (4th ed.). New York: Oxford University Press. pp. 911–932.
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9. ^ Molgula pugetiensis is a Pacific tailless ascidian within the Roscovita clade of molgulids
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12. ^ Colonial Allorecognition, Hemolytic Rejection, and Viviparity in Botryllid Ascidians
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14. ^
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15. ^
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29. ^ "Pyura sp. Evolution and Systematics". University of Queensland. Retrieved 3 April 2017.
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34. ^ Gold, Jonathan (26 November 2008). "Supersuckers: Masan's Tenacious Tentacles and Fiery Monkfish". LA Weekly. Retrieved 29 March 2017.
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General references

• ISBN 0-19-860426-2
  .

External links

Wikimedia Commons has media related to Ascidiacea.

• The Dutch Ascidians Homepage
• Encyclopedia of Marine Life of Britain and Ireland
• A fate map of the ascidian egg
• Ciona savignyi Database
• ANISEED Ascidian Network for In Situ Expression and Embryological Data
• Photos of Ascidiacea on Sealife Collection